1. Field of the Invention
This invention relates to a hollow fiber membrane for an artificial lung, a method for the manufacture thereof, and the artificial lung using the hollow fiber membrane. Particularly, this invention relates to a porous hollow fiber membrane possessing a high gas-exchange capacity, a method for the manufacture thereof, and an artificial lung using the hollow fiber membrane. More particularly, this invention relates to a porous hollow fiber membrane capable of retaining a high gas-exchange capacity during a protracted use without entailing any leakage of blood plasma and suitable for use in an artificial lung, a method for the manufacture thereof, and the artificial lung using the hollow fiber membrane.
2. Description of Prior Art
Generally in the surgical operation of the heart, for example, an artificial lung incorporating therein hollow fiber membranes is used in an external circulatory path for the purpose of leading the patient's blood out of his circulatory system, adding oxygen to the blood, and stripping the blood of carbon dioxide gas. The hollow fiber membranes offered for use in the artificial lung of this nature are broadly divided into two types; homogeneous membranes and porous membranes. The homogeneous memebranes permit the permeating gas to keep its motion by enabling the molecules of the gas to dissolve and diffuse in the membrane. A typical example of these homogeneous membranes is one made of silicone rubber which has been commercialized as Mela Silox (manufactured by Senko Ika Kogyo K.K.). Because of gas permeability considerations, however, it is only this type of homogeneous membrane of silicone rubber that has been accepted as fully serviceable to date. However, strength considerations prevent this homogeneous membrane of silicone rubber from having a thickness smaller than 100 .mu.m. Thus, it has limited permeability to gases and exhibits particularly poor permeability to carbon dioxide gas. Moreover, the silicone rubber suffers from a disadvantage that it is expensive and is difficult to work.
The porous membranes permit the permeating gas to move through their pores as volume flow because the pores are notably large as compared with the molecules of the gas. Various artificial lungs have been proposed which use porous membranes such as microporous polypropylene membranes. For example, it has been proposed to manufacture a porous polypropylene hollow fiber by melt spinning polypropylene with a hollow fiber producing nozzle at a spinning temperature of 210.degree. to 270.degree. C. and a draft ratio of 180 to 600, then subjecting the extruded fiber to a first heat treatment at a temperature of not higher than 155.degree. C., stretching the fiber at a temperature lower than 110.degree. C. at a ratio of 30 to 200%, and thereafter subjecting the stretched fiber to a second heat treatment at a temperature exceeding the treatment of the first heat treatment and not exceeding 155.degree. C. (Japanese Patent Publication SHO 56(1981)-52,123). In the porous hollow fiber obtained as described above, however, the pores thereof have been physically formed by stretching the hollow fiber of polypropylene. The pores formed therein, therefore, these pores are linear pores substantially horizontal in the direction of membrane thickness. Further since the pores take shape while producing cracks in the axial direction of the hollow fiber proportionately to the degree of stretching, their cross sections resemble slits. The pores continuously penetrate the wall of the hollow fiber substantially linearly and have a high porosity. As the result, the porous hollow fiber exhibits high permeability to moisture and has a disadvantage that it suffers leakage of blood plasma after a protracted use in the external circulation of blood.
As a porous membrane incapable of leakage of blood plasma, there has been proposed a porous polyolefin hollow fiber membrane which is produced by mixing a polyolefin, an organic filler capable of uniformly dispersing in the polyolefin in a fused state and easily dissolving in an extractant being used, and a crystal seed forming agent, discharging the resulting mixture in a molten state through an annular spinning nozzle and, at the same time, introducing an inert gas into the interior of the hollow fiber of the mixture, allowing the hollow fiber to contact a cooling and solidifying liquid incapable of dissolving the polyolefin, and then allowing the cooled and solidified hollow fiber to contact an extractant incapable of dissolving the polyolefin thereby extracting the organic filler from the hollow fiber (Japanese Patent Application SHO 59(1984)-210,466). The polypropylene hollow fiber membrane obtained by using a cooling and solidifying liquid to dissolve the organic filler being used does not suffer any leakage of blood plasma because the pores are small and the paths of these pores are complicated. However, this structure has the disadvantage, when used in an artificial lung, of insufficient gas-exchange capacity because it has a small pore density per unit area. Further, the polyolefin hollow fiber membrane has a possibility that the low molecular component of the polyolefin will mingle into the cooling and solidifying liquid capable of dissolving the organic filler, adhere to the inner wall of the coolant tube, and deform the hollow fiber on aging.
This invention, therefore, has an object to provide an improved hollow fiber membrane for use in an artificial lung, a method for the manufacture thereof, and an artificial lung using the hollow fiber membrane.
This invention also aims to provide a porous hollow fiber membrane possessing a high gas-exchange capacity, a method for the manufacture thereof, and an artificial lung using the hollow fiber membrane.
This invention further aims to provide a porous hollow fiber membrane made of polypropylene capable of retaining a high gas-exchange capacity during a protracted use without entailing any leakage of blood plasma and suitable for use in an artificial lung, a method for the manufacture thereof, and an artificial lung using the hollow fiber membrane.